An air spring has layers of reinforcement cords which run parallel and are embedded in rubber. These reinforcement cords are also known as reinforcement layers which take up the forces which arise because of the overpressure in the air spring flexible member. The reinforcement cords are, in general, so-called cord threads wherein several individual fibers (filaments) are twisted into a filament-shaped formation. The reinforcement threads preferably comprise full-synthetic fibers which exhibit a high capability of bending, strength and a uniform stretching capability. These fibers are made, for example, of polyester, polyamide or aramide and can also be made of metal.
In the usual method for producing air springs, the individual unvulcanized components of the air spring (inner rubber, reinforcement layers, outer rubber and, if required, wire cores) are wound or manufactured to an air spring blank and this blank is then vulcanized. In this method, as a rule, rubberized cord fabric layers are used for the reinforcement layers. The cord fabric comprises a plurality of parallel cord threads lying one next to the other and lying in the warp direction. The cord threads are loosely held together at greater spacings by a few thin threads in the direction of the weft. After an impregnation operation for increasing the adherence of rubber (fabric preparation), these cord fabrics are rubberized with a suitable rubber mixture, for example, a mixture on the basis of chloroprene rubber and, after the diagonal cutting into the wanted layer number, are so wound on the blank that the cords of the various layers come to rest crosswise, preferably, at a cord angle of 40° to 80° to the peripheral direction. With the cord angle, and for a given air spring contour, carrying force and lateral force of the air spring can be influenced.
Another method for manufacturing air springs is described, for example, in DE 198 46 852 A1. In this method, cord-reinforced tubular blanks are continuously manufactured by the spiral-formed wrapping of a coated mandrel with reinforcement cords. The tubular blanks comprise several layers and these blanks are, for example, tubular rolling-lobe flexible members for air springs. Overlapping regions and abutting regions are avoided with this method.
Newer developments of motor vehicles or chassis, especially in the automobile area, call for increasingly smaller air spring types having correspondingly increasing operating pressures. Especially high requirements are imposed on the layers of reinforcement cords in order to withstand these pressures, that is, to realize a highest possible bursting pressure while at the same time ensuring a high bending strength and therefore to ensure a high service life of the air springs. The number of layers may not be too great so that a high flexibility can be achieved. At the same time, the strength and the filament thickness of the reinforcement cords must be so selected that the air spring achieves a bursting pressure as high as possible. If the spacing of the cords becomes too great, then the danger is present that the rubber material is not sufficiently burst-tight and a point-for-point destruction of the air spring in the clear distances between cords could occur.
In U.S. Pat. No. 5,566,929, an air spring flexible member is described which exhibits a high stand time at high inner pressures and low roll-off radii in the rolling lobe. This rolling-lobe flexible member has, as a reinforcement layer, three rubberized cord fabric layers one atop the other. The middle cord fabric layer has a layer strength which corresponds to the sum of the layer strengths of the first and third layers. The fabric within one and the same layer comprises uniform cord filaments, that is, the cord filaments are made of the same material and have identical cord construction.